I personally do not believe that sports teams should be considered for this 'limit' it should be two factions, and then you can ALSO join a sports team.. Personally..
But all together -1
Honestly, I agree with your view on sports! The reason I mentioned sports teams being apart of a faction is already because of the idea that they are a section of players that, once joined, take up a spot in the primary due to the nature of the role. Seeing as how the spot takes a title on the primary, I'd file it in with the other factions that follow the similar path.
I definitely would argue that sports shouldn't fall under that same category, I believe that the process for sports, tryouts and skill towards a sports plugin, is a very different application process than for... well the other factions. But, that being said, the basis of my suggestion really is about pressing the importance of keeping that per person limit for fairness, opportunities, and the things I've mentioned before. There are work arounds behind it all, of course!
One way to work around it would be to remove the faction tag for sports entirely.
CRAZY I KNOW BUTTT- hear me out! I saw on another suggestion that people in clubs are differentiated through the use of a tag in their description, I haven't seen that before but it is a thing! If we wanted to move sports into a format where we then made their special role into a similar style of label, we could avoid the issue of [Sports Team] being a tag that is then placed on the primary. With that, one person would still be allowed to join a sports team on a secondary character, while also then using their (now free!) primary spot to apply themselves into a faction!
Another way could be to code the role of sports faction into a secondary slot position, but we've already seen that that's a coding issue that's difficult to get by.